Gamma prime strengthened nickel-base superalloys are essential for critical components in aero and land based gas turbines, but, are used also in other applications. The difference between said superalloys depend on the level of knowledge and production technology available at the time they were developed, and, on different relative emphasis on properties such as hot corrosion resistance, oxidation resistance, coating compatibility, phase stability, creep strength and density.
Nickel-base gamma prime strengthened superalloys are used in monocrystalline, directionally solidified or equiaxed form. In each crystal there is a matrix of the phase gamma which is essentially Ni with elements like Co, Cr, Mo, W and Re in solid solution, and, particles of the phase gamma prime which is essentially Ni3Al with elements like Ti, Ta, Nb and V in solid solution. Grain boundaries, if present, are usually decorated by carbides and/or borides which provide cohesive strength. Zr and Hf also contributes to grain boundary cohesion.
Creep strength is provided by the elements Mo, W and Re which provide solution strengthening to the gamma matrix, and, Ti, Ta, Nb and V which provide solution strengthening to the gamma prime particles. Ta has a particularly high strengthening effect per at %. Also, Al provides creep strength because it increases the amount of gamma prime particles, and, because it concentrates the levels of Mo, W and Re in the matrix.
If the concentration of Cr, Mo, W and Re in the matrix is too high, so called topologically close packed (TCP) phases will form in service. Therefore, an increased gamma prime content, or, increased Mo, W or Re levels, must be accompanied by a reduction in Cr if extensive TCP formation is to be avoided. One particular effect of TCP precipitation is a reduction in creep strength.
[Caron 1] teaches that the tendency for TCP precipitation for an alloy can be estimated through comparison of the Md value for this alloy and a relatively similar alloy with known TCP risks. For the alloys analyzed here, the Md value can be calculated fromMd=0.717aNi+0.787aCo+1.142aCr+1.55aMo+1.655aW+1.9aAl+2.271aTi+2.224aTa
wherein aCo is the content of Co in atom % etc.
Hot corrosion resistance is provided by Cr, and, the classical rule is that at least 12 wt % Cr is needed for adequate hot corrosion resistance. It is also important to allow at most moderate Mo levels.
[Goldschmidt] teaches that the hot corrosion resistance of the alloy SC16 with 16 wt % Cr and 3 wt % Mo is inferior to that of IN738LC with 16 wt % Cr and 1.8 wt % Mo. IN738LC is generally recognized as having a high hot corrosion resistance. Therefore a restriction to at most 2 wt % Mo in a new alloy seems prudent.
In the context of high firing temperature gas turbines it is generally accepted that high oxidation resistance require the ability to form an adherent continuous AL2O3 scale, as needed for metal temperatures on the 1000 degree Celcius level.
[Barrett] teaches that this ability is provided by Al, enhanced by Cr and Ta, somewhat reduced by Mo and W, reduced by Ti and Nb, and significantly reduced by V. This implies that less Al is needed to form such an Al2O3 scale if the levels of Cr and Ta are increased, or, the levels of Ti, Nb and V are reduced.
[Sarioglu] teaches that the scale adherence is severely reduced by tramp elements such as S, but, that this effect can be neutralized by a combination of clean casting and addition of small measured levels of reactive elements (RE) such as Zr, Hf and rare earths.
[Pint 1] underlines the importance of S, and further teaches the beneficial RE effects when small levels of Hf and the rare earth Y are combined.
[Caron 2] teaches the beneficial RE effects when small levels of Hf and Si are combined.
[Pint 2] teaches that optimal RE effects can be obtained when multiple RE are used, one example being the excellent cyclic oxidation resistance seen in tests on Haynes-214 which contained small levels of Zr, Si and Y.
One important aspect of coating compatibility particularly relevant for modern high firing temperature gas turbines is the cyclic life of an applied theimal barrier coating.
[Wahl&Harris] teaches that the spallation life of a TBC increased significantly when rare earths were added to the base alloy.
[Wu] teaches that the spallation life of a TBC can be correlated to the oxidation resistance of the base alloys. In particular, the lowest spallation life was obtained for the base alloy with the highest Ti content.
The density is reduced by the light elements Al and Ti, and increased by the heavy elements W, Re and Ta.
[Caron 1] teaches that for the alloys of interest for us, the density in kg/dm3 can be calculated fromDensity=8.29604−0.00435aCo−0.0164*aCr+0.01295aMo+0.06274aW−0.06595aAl−0.0236aTi+0.05441aTa
wherein aCo is the content of Co in atom % etc.
The used references can be found as follows:
[Caron 1] P. Caron High Gamma Prime Solvus New Generation Nickel-Based Superalloys for Single Crystal Turbine Blade Applications Proceedings ‘Superalloys 2000’
[Goldschmidt] D. Goldschmidt Single-Crystal Blades Proc. from Materials for Advanced Power Engineering 1994, Part I, p. 661-6741
[Barrett] C. A. Barrett A Statistical Analysis of Elevated Temperature Gravimetric Cyclic Oxidation Data of 36 Ni- and Co-base Superalloys based on an Oxidation Attack Parameter NASA™ 105934
[Sarioglu] C. Sarioglu, et al. The Control of Sulfur Content in Nickel-Base Single Crystal Superalloys and its Effect on Cyclic Oxidation Resistance Proceedings ‘Superalloys 1996’
[Pint 1] B. A. Pint et al Effect of Cycle Frequency on High-Temperature Oxidation Behavior of Alumina- and Chromia-Forming Alloys Oxidation of Metals, 58 (1/2), 73-101 (2002)
[Caron 2] P. Caron et al. Improvement of the Cyclic Oxidation Behaviour of Uncoated Nickel Based Single Crystal Superalloys Materials Proceedings ‘Materials for Advanced Power Engineering 1994’
[Pint 2] B. A. Pint et al. The use of Two Reactive Elements to Optimize Oxidation Performance of Alumina-Forming Alloys Materials at High Temperature 20(3) 375-386, 2003
[Wahl&Harris] J. B. Wahl, K. Harris Advances in Single Crystal Superalloys—Control of Critical Elements Proceedings ‘7th Parsons conference’, 2007
[Wu] R. Wu et al On the Compatibility of Nickel-Based Single Crystal Superalloys with Coating Systems Proceedings ‘7th Parsons conference’, 2007
[Caron 3] P. Caron et al Development of New High Strength Corrosion Resistant Single Crystal Superalloys for Industrial gas Turbine Applications Proceedings ‘5th Parsons conference’, 2000
Early alloy development resulted in alloys such as IN713LC which has a composition, in wt %, given by Ni-12Cr-4.5Mo-6Al-0.6Ti-4Ta-0.1Zr-0.05C-0.01B, a particle content of about 55 vol %, and a low density of about 8.0 kg/dm3. Reliance on Mo for matrix strengthening is typical for these early alloys.
6 wt % Al supported by 12 wt % Cr, 4 wt % Ta and a reactive element effect from Zr enable high oxidation resistance. Despite 12 wt % Cr, the hot corrosion resistance is poor due to the high Mo level. The density is low thanks to low levels of heavy elements.
One line of subsequent alloy development led to alloys such as IN792 which has a composition, in wt %, given by Ni-9Co-12.5Cr-1.8Mo-4.2W-3.4Al-4.2Ti-4.2Ta-0.08C-0.015B, a particle content of about 50 vol %, and a moderate density of 8.25 kg/dm3.
Compared to IN713LC, Mo is partly replaced by W for improved hot corrosion resistance, and, Al is partly replaced by Ti for improved solution strengthening of the particles. High Ti levels is typical for these alloys.
Replacement of Mo by W increases the density. Due to the partial replacement of Al by Ti these alloys are unable to form Al2O3 scales, therefore they do not provide high oxidation resistance.
Another line of subsequent alloy development led to alloys like CMSX-4 which has a composition, in wt %, of Ni-9Co-6.5Cr-0.8Mo-6.5W-3Re-5.65Al-1.2Ti-6Ta-0.1Hf, a particle content of about 70 vol %, and a high density of 8.67 kg/dm3. These alloys combine very high particle contents with very high levels of matrix strengthening elements, which has forced the Cr levels to very low levels to avoid TCP precipitation.
High levels of Al and Ta and low levels of Ti and Nb enable high oxidation resistance and good coating compatibility despite low Cr levels. Significant work has been done to further enhance their oxidation resistance and coating compatibility via clean casting and use of RE effects. Their hot corrosion resistance is poor due to their low Cr levels.
A further line of alloy development led to alloys such as CMSX-6 which has a composition, in wt %, given by Ni-5Co-10Cr-3Mo-4.8Al-4.7Ti-2Ta-0.1Hf, a particle content of about 60 vol %, and a very low density of 7.83 kg/dm3.
These alloys are characterized by high levels of Al and Ti, less than 12 wt % Cr, and reliance on Mo rather than Re or W for matrix strengthening. The combination of less than 12 wt % Cr, and, reliance on only Mo for matrix strengthening, result in a less than adequate hot corrosion resistance. Furthermore, the high Ti levels prohibit high oxidation resistance and coating compatibility despite relatively high Al levels.